Sharapova was tested for the first time, but still emerged with a 6-1, 6-2 win over Germany’s Angelique Kerber.

The 2008 champion has lost five games in reaching the fourth round. She won her first two matches 6-0, 6-1 despite not playing any warmup tournaments because of an ankle injury.

The scoreline on Friday made it look easy enough for Sharapova, but she was pushed during a 56-minute second set, with many games going to deuce.

Sharapova and Kvitova both have a chance of claiming the No. 1 ranking at the end of the tournament. They could play each other in the semifinals, although Kvitova insisted she hasn’t looked that far ahead.

“I don’t know who lost and who win,” the Wimbledon champion said. “No, really, for me doesn’t care.”

Kvitova reached the round of 16 when Maria Kirilenko retired with a left thigh injury while trailing 6-0, 1-0 after 38 minutes of their third-round match.

Kvitova next faces Ana Ivanovic, who beat unseeded American Vania King 6-3, 6-4 to reach the fourth round for the first time since she reached the final here in 2008.

That was the same year the 24-year-old Serb won the French Open for her only Grand Slam title, and also claimed the top ranking.

King’s loss left Serena Williams as the only American player left in the singles draws after John Isner’s loss to Feliciano Lopez in five sets on Thursday ended any hope of a men’s champion from the United States.

Five-time champion Williams, hoping to win her 17th straight match at Melbourne Park, next was scheduled to play Greta Arn of Hungary.